A conventional 50 watts transformer for converting an input voltage of 220 volts AC to an output voltage of 110 volts AC may serve as a traveller's voltage converter for portable uses. Since the 50 watts (W) transformer may only be provided for those electronic or electric appliances such as portable personal computer having a load wattage less than 50 watts, an uncareful using of such a 50 W low-load transformer for those electric appliances of higher load up to 1600 W, such as: hair dryer, coffee maker or travel iron, may burn out the transformer or the fuse to lose the power transforming function, possibly causing inconvenience especially when travelling in European and Middle East areas.
A conventional 1600 watts traveller's transformer for converting 220 volts (AC) to 110 volts (AC) may be provided for powering the electric appliances with power rating from 50 watts to 1600 watts such as for uses in those appliances requiring higher wattage such as hair dryer, travel iron and coffee maker. It includes a bidirectional diode thyristor (DIAC) and a bidirectional triode thyristor (TRIAC) for stepping down the voltage from 220 volts to 110 volts. However, the peak voltage output from such a 1600 W transformer is 220 VAC, rather than the reduced voltage value of 110 VAC, still possibly damaging the electronic appliance containing integrated circuit (IC), transistors and other electronic parts vulnerable to high voltage of 220 volts.
Naturally, a combination of 50 W transformer with a 1600 W transformer may be derived from the aforementioned two transformers, which may be selected for either 50 watts load or 1600 watts depending upon the end-use requirement by manipulating a selector rotatably mounted in a casing of the transformers. However, a false or careless manual selection may burn out the electronic appliance if accidentally selecting the power rating of 1600 W; or may destroy the 50 W transformer if selecting the power rating of 50 W while adopting a load larger than 50 W.